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16    BAiRE'S THEOREM AND ITS CONSEQUENCES       91

Let x0 e E. We have to show that, for each neighborhood V of e in G, the
set V  *0 is a neighborhood of *0 in E (12.11.5). Let W be a compact symmetric
neighborhood of e in G such that W2 c: V, and let (sn) be a sequence which
is dense in G, such that (jflW) is a covering of G. Each of the sets snVi  x0 is
closed in E, because s^s- x0 is continuous (12.3.6), and E is the union of the
denumerable sequence of closed sets sn W  x0. By Baire's theorem (12.16.1),
there exists an index n such that 5-nW  x0 has an interior point sns- x0, with
s e W. It follows (12.10.3) that x0 is an interior point of the set

in other words, V  x0 is a neighborhood of x0.                                 Q.E.D.

(12.16.13) Let Gbea separable, metrizable, locally compact group, let G' be a
metrizable group, and letf: G -* G' be a continuous surjective homomorphism.
Then f is a strict morphism (12.12.7) (in other words, if H is the kernel of/
then the canonical bijection g: G/H -> G' is an isomorphism of topological
groups).

For we may consider G' as a space on which G acts continuously and
transitively by the rule (s9 t')i-*f(s)t', and the stabilizer of the neutral element
e1 of G' is H. The result therefore follows from (12.16.12).

PROBLEMS

1.    Let E, F be two metric spaces, A a dense subspace of E, and/a continuous mapping
of A into F. If F is complete, show that the set of points in E at which/has no limit
relative to A (3.13) is meager in E. (For each n, consider the set of points xe E at
which the oscillation of/with respect to A (3.14) is >!//?.)

2.    Let E, F be two complete metric spaces, and/a homeomorphism of a dense subspace
A of E onto a dense subspace B of F. Show that there exists a subspace C ^> A in E
(resp. D ^ B in F) which is a denumerable intersection of open sets, and an extension
of/to a homeomorphism g of C onto D. (Apply Problem 1 to/and its inverse.)

3.    Let E be a complete metric space, F a metric space, and (/) a sequence of continuous
mappings of E into F which converges simply (pointwise) in E to a mapping/. Show
that the set of points x e E at which /is not continuous is meager in E. (Let Gp,n
be the set of x e E for which the distance between fp(x) and f^(x) is <l/2n for all
q >p. Show that the union of the interiors of the sets Gp,  for p 2> 1 is a dense open
set, by using Baire's theorem. Deduce that the set of points at which the oscillation of
/is ^l/n contains a dense open set.)

Generalize to the case where it is assumed only that, for each /?, the restriction of
fn to the complement of a meager set M (depending on n) is continuous,t, for each contraction U e tf and each vector x e E, the